We now consider three hydrogen line ratios. The intrinsic hydrogen line ratios have been controversial since the first spectrophotometry of AGNs in the mid-1960s. This controversy, which has been entwined with the AGN reddening question, is reviewed in [1]}. In the 1960s, low Ly\(\) /H\(\)  and high H\(\alpha \) /H\(\)  ratios were found which differed from the ratios expected from Case B values of recombination theory [2]}, [3]}. [2]} noted that reddening of \(E(B-V) \sim 0.2\)  could explain the Balmer decrement of Ton 1542. He also estimated the Ly\(\alpha \) /H\(\alpha \)  ratio as \(\approx 2.7\)  by comparing high- and low-redshift AGNs [3]}. This was much less than the Case B prediction. Although reddening offered a solution to the differences from Case B, theoretical calculations by many research groups showed that some of the observational results could be explained by radiative transfer and collisional effects instead, but it was difficult to obtain a coherent picture. [1]} presented evidence that, after de-reddening, velocity-integrated broad-line hydrogen line ratios in AGNs were consistent with Case B recombination values. He pointed out that optically-thick, one-dimensional-slab radiative transfer models that produce values differing from Case B were unlikely to be relevant to real AGNs because the line radiation would escape from clouds sideways.
